1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device, and particularly relates to a semiconductor integrated circuit device including a combinational logic circuit.
2. Related Background Art
Uvighara, et al. show a leakage current reducing method in ISSCC2004 Digest of Technical Papers (23.3) February 2004 and Slide Supplements (23.3) [Non-Patent Document 1].
In the leakage current reducing method disclosed in Non-Patent Document 1, two types of logic cells, a normal logic cell and a logic cell with a foot switch, are prepared. The normal logic cell includes only high threshold voltage (HVT) transistors, but the logic cell with the foot switch includes a standard cell formed by a low threshold voltage (LVT) transistor and the foot switch formed by a high threshold voltage (HVT) transistor.
In this case, the logic cell with the foot switch can operate at higher speed than the normal logic cell including only the high threshold voltage transistors, but the leakage current while the foot switch is on is larger than that of the normal logic cell.
In Non-Patent Document 1, a combinational logic circuit is formed by a mixture of these normal logic cell and logic cell with the foot switch. On this occasion, the use of the normal logic cell is tried as much as possible, and only when the speed requirement is not met, the logic cell with the foot switch is used.
Generally, there are plural clock domains inside a semiconductor integrated circuit. Power consumption is reduced by stopping the supply of a clock signal of a clock domain corresponding to a portion which need not be operated, and this technique is called gated clock or clock gating.
The control whether the clock signal is supplied to a specific clock domain is performed by a control circuit provided in the middle of a clock tree. When the supply of the clock signal is stopped, a combinational logic circuit of its corresponding clock domain is stopped, and consequently if a logic cell with a foot switch is provided in this combinational logic circuit, the leakage current can be reduced by turning off this foot switch. To turn on/off the foot switch of the logic cell with the foot switch, the control circuit is required to supply its control signal to the foot switch.
However, if the foot switch of the combinational logic circuit is turned off before the combinational logic circuit finishes an operation, a proper operation result is not sometimes obtained, and therefore, also in Non-Patent Document 1, the foot switch is turned off several cycles after the supply of the clock signal is stopped, and the clock signal is supplied several cycles after the foot switch is turned on.
In Non-Patent Document 1, the number of delay cycles is ex post adjustable. More specifically, it suggests setting the number of delay cycles after manufacturing since it is difficult to estimate the required number of delay cycles in the design phase of the combinational logic circuit. This is because the setting of the number of delay cycles corresponds to that of a waiting time during operation of the integrated circuit, so that setting with minimum expenditures of time is desired.
As can be seen from the above, Non-Patent Document 1 has three main problems. The first problem is that a relatively long delay cycle is needed for switching between active and standby states where the clock signal is supplied/stopped, respectively, and to control this, complicated hardware is necessary. The second problem is that it is difficult to estimate the number of delay cycles required to obtain the proper operation result from the combinational logic circuit in the design phase, and hence the adjustment of the number of delay cycles is needed after manufacturing. The third problem is that the number of delay cycles to be set may vary from product to product due to variations in manufacturing process, so that the burden required for its adjustment becomes extremely large.